The present system relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system providing, among other things, a differential sensing channel.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is in acquiring the intrinsic electrical signals produced by the heart, which are referred to as xe2x80x9ccardiac signalsxe2x80x9d or xe2x80x9cheart signals.xe2x80x9d Such cardiac signals include, among other things, electrical depolarizations associated with heart chamber contractions and electrical repolarizations associated with heart chamber expansions. Decisions regarding the delivery and/or withholding of cardiac rhythm management therapy are often based at least in part on information included within such cardiac signals. Moreover, cardiac signals may include information indicating the efficacy of therapy that has already been delivered. Therefore, acquisition of cardiac signals is one important task of implantable or external cardiac rhythm management systems.
One type of circuit used in acquiring cardiac signals includes a linear preamplifier, receiving signals from first and second cardiac electrodes and providing a single-ended output signal based thereon. The linear preamplifier is followed by a switched-capacitor signal processing channel that performs single-ended discrete time signal processing including analog-to-digital conversion. Switched-capacitor signal processing circuits perform signal processing using charges that are sampled onto capacitors during a xe2x80x9cclock period.xe2x80x9d Such a signal processing circuit, however, presents several drawbacks that limit its usefulness in an implantable cardiac rhythm management device, as discussed below.
For example, a switched-capacitor filter circuit utilizes an operational transconductance amplifier (OTA) that typically has an intrinsic offset voltage. The offset voltage may introduce errors into cardiac signals being processed. In order to accommodate such offset voltages and to decrease the effect of offset errors, autozeroing circuits are typically used in the single-ended signal processing channel. An autozeroing circuit works by periodically prestoring a charge associated with the offset voltage on a signal processing capacitor (i.e., xe2x80x9cautozeroingxe2x80x9d). Then, subsequent single-ended signal processing is performed using differences between the prestored charge and the charge associated with the cardiac signal or other desired signal of interest.
Autozeroing is typically performed during the same length of time that is allocated for the clock period during which the cardiac signal is sampled. Moreover, during autozeroing the gain and feedback configuration of the operational amplifier and the load capacitance being driven typically impose more strenuous demands on the amplifier than does normal sampling of the cardiac or other signal being processed. As a result, proper autozeroing operation of the operational amplifier typically dictates the magnitude of the bias current used in the operational amplifier. However, in power-sensitive applications, such as battery-powered implanted medical devices, larger currents cause the battery to drain faster. This reduces the useful life of the implanted device. A shorter device life results in more frequent replacement of the implanted medical device, with all its attendant costs and risk of complications. Thus, there is a need for improved autozeroing and/or other cardiac signal processing techniques that consumes less power, in order to provide more economical medical treatment, and to reduce the risks associated with replacement of an implanted cardiac rhythm management or other implanted medical device.
This document describes, among other things, portions of a differential signal processing channel for differentially processing signals, such as cardiac signals in an implantable cardiac rhythm management device. The differential signal processing channel offers many advantages over single-ended discrete-time processing of cardiac signals using conventional autozeroing techniques. For example, the above-described differential signal processing techniques allow the use of lower bias currents in the active circuits used in discrete time signal processing. This saves power, thereby extending the useful life of the implanted device before explantation and replacement becomes necessary. The differential signal processing techniques also reduce the effects of clock feedthrough of the switches used in the switched-capacitor circuits, provide a wider dynamic range for the cardiac signals being acquired, and provide better rejection of power supply noise. This document also describes an operational transconductance amplifier (OTA), capable of use in the discrete-time differential signal processing channel. The OTA provides, among other things, an output common mode adjustment circuit and an offset compensation circuit.
In one embodiment, a cardiac rhythm management system includes an implantable cardiac rhythm management (CRM) device. The CRM device includes an interface circuit, including first and second inputs and including first and second outputs providing an analog differential output signal based on the signals at the first and second inputs. The CRM device also includes a discrete-time differential signal processing channel providing a digitized output signal based at least in part on an analog differential output signal received from the first and second outputs of the interface circuit.
This document also discloses an operational transconductance amplifier (OTA) circuit capable of use in the discrete-time differential signal processing channel. The OTA circuit includes a differential-voltage-to-differential-current converter circuit, a common mode adjustment circuit, and an offset correction circuit.
This document further discloses a method of acquiring cardiac signals. Cardiac signals are received at first and second cardiac electrodes. The cardiac signals arc communicated to an implantable cardiac rhythm management device. The cardiac signals received from the first and second cardiac electrodes are buffered to provide a differential buffered signal. The differential buffered signal is processed differentially and in discrete-time. A digitized output signal is provided based on the processed differential signal.
In one embodiment, the disclosed signal processing methods include receiving a differential input voltage between first and second inputs and providing a differential output current, based on the differential input voltage, at first and second outputs. A common mode voltage at the first and second outputs is adjusted. An offset in the differential output current at the first and second outputs is compensated. These and other aspects of the present system and methods will become apparent upon reading the following detailed description and viewing the accompanying drawings that form a part thereof.